Ion-exchange composite membranes (IEM) are used in fuel cells as solid electrolytes. A membrane is located between the cathode and anode and transports protons formed near the catalyst at the hydrogen electrode to the oxygen electrode thereby allowing a current to be drawn from the cell. These polymer electrolyte fuel cells are particularly advantageous because they operate at lower temperatures than other fuel cells. Also, these polymer electrolyte fuel cells do not contain any corrosive acids which are found in phosphoric acid fuel cells.
Ion-exchange composite membranes are also used in chloralkali applications to separate brine mixtures to form chlorine gas and alkali hydroxide. The membrane selectively transports the alkali metal ions across the membrane while rejecting the chloride ions.
Additionally, IEMs are useful in the areas of diffusion dialysis, electrodialysis and for pervaporation and vapor permeation separations.
In electrodialysis, electrolytes can be divided into a concentrated and a diluted stream. This can be accomplished by arraying anionic and cationic exchange membranes in a filter press arrangement. Alternating compartments between the membranes are filled with either the feed stream or the product stream. An electric field is applied across this series array by inserting electrodes in the end compartments.
In diffusion dialysis, a stream of contaminated acid or base can be separated from dissolved metal ions, colloidal or non-ionic species. The acid or base can than be returned to the original process. A diffusion dialysis system may consist of a filter press type arrangement with anion or cation exchange membranes between compartments of that system. Alternate compartments are filled with either the waste material or water. The desired ions diffuse through the membrane. The undesired ions are rejected and removed as waste.
IEMs must have sufficient strength to be useful in their various applications. For example, IEMs that are not reinforced (such as those commercially available from E. I. DuPont de Nemours, Inc., and sold under the registered trademark Nafion.RTM.) are inherently weak. Often the need for increased strength requires the membranes to be made thicker which decreases their ionic conductance.
As seen below, fluorinated ion-exchange polymers having carboxylic acid and/or sulfonic acid functional groups, and salts thereof, are known, as is their use in fuel cells and chloralkali electrolysis cells and the like. In such uses, the polymers are generally used as one layer of a membrane laminate. Multi-layer laminates suffer from delamination under certain operating conditions.
U.S. Pat. No. 4,469,744 to Grot et al. uses such polymers in a protective clothing of fabric containing a layer of the highly fluorinated ion-exchange polymer. Example 1 refers to a microporous polytetrafluoroethylene film having a thickness of 127 micrometers as described by U.S. Pat. No. 3,962,153. A solution of ion-exchange polymer is applied with the use of a vacuum. The film was then placed in an oven (under vacuum) at 120.degree. C. for 5 hours. The final product had a thickness of about 127 micrometers (5 mils) and required the use of a vacuum to provide for impregnation of polymer.
U.S. Pat. No. 4,902,308 to Mallouk, et al. relates to a film of porous expanded PTFE having its surfaces, both exterior and internal, coated with a metal salt of perfluoro-cation exchange polymer. The base film of porous, expanded PTFE (ePTFE) had a thickness of between 1 mil and 6 mils (0.025-0.150 mm). The final composite product had a thickness of at least 1 mil (0.025 mm) and preferably had a thickness of between 1.7 and 3 mils (0.043-0.075 mm). The composite product was permeable to air and the air flow, as measured by the Gurley densometer ASTM D726-58, was found to be between 12 and 22 seconds. Therefore, this structure allows fluid to pass through.
U.S. Pat. No. 4,954,388 to Mallouk, et al. relates to an abrasion-resistant, tear resistant, multi-layer composite membrane having a film of continuous ion-exchange polymer attached to a reinforcing fabric by means of an interlayer of porous expanded PTFE. A coating of a ion-exchange resin is present on at least a portion of the internal and external surfaces of the fabric and porous expanded PTFE. The composite membrane made in accordance with the teachings of this patent resulted in thicknesses of greater than 1 mil (0.025 mm) even when the interlayer of porous expanded PTFE had a thickness of less than 1 mil (0.025 mm).
U.S. Pat. No. 5,082,472 to Mallouk, et al. relates to a composite membrane of microporous film in laminar contact with a continuous ion-exchange resin layer wherein both layers have similar area dimensions. Surfaces of internal nodes and fibrils of porous expanded (ePTFE) may be coated at least in part with ion-exchange resin coating. The membrane of ePTFE can have a thickness of about 2 mils (0.050 mm) or less and the ion-exchange layer can have a thickness of about 2 mil. The ePTFE layer of this composite membrane imparted mechanical strength to the composite structure. While the patent says the pores can be filled with the ion-exchange resin, they cannot be filled to the point where flow of fluids is blocked.
U.S. Pat. Nos. 5,094,895 and 5,183,545 to Branca, et al. relate to a composite porous liquid-permeable article having multiple layers of porous ePTFE bonded together and having interior and exterior surfaces coated with an ion-exchange polymer. This composite porous article is particularly useful as a diaphragm in electrolytic cells. The composite articles are described to be relatively thick, preferably between from 0.76 and 5 mm.
U.S. Pat. No. 5,547,551 filed Mar. 15, 1995 to Bahar, et al., describes an ion-exchange membrane of expanded porous PTFE less than 0.025mm (25 microns) in which the pores are fully impregnated with an ion-exchange material.
Japanese Patent Application No. 62-240627 and Japanese Patent No. 5-75835 relate to a coated or an impregnated membrane formed with a perfluoro type ion-exchange resin and porous PTFE film to form an integral unit. The resulting composite does not appear to be fully occluded with resin since the patent speaks only of thorough impregnation of resin solution.
None of the above described materials adequately address the current and anticipated demands for an ion-exchange membrane. There remains a distinct need for a strong, ultra-thin, integral composite ion-exchange membrane, having long term chemical and mechanical stability, very high ionic conductance, and being thin. For example, fabric reinforced ion-exchange membranes result in thick composites, which leads to operation at high voltages because the greater thickness means higher electrical resistance. Trying to make the membrane thinner only results in weaker, tear-prone membranes.